The present invention relates to a photoconductor for electrophotography. More specifically, the present invention relates to a photoconductor for electrophotography useful in printers, copiers, and the like, having improved additives in an organic charge transport layer. Even more specifically, the present invention provides a photoconductor having a small variation in the charged potential between the initial charged potential and the charged potential after repeated use. The present invention further provides a photoconductor having a small variation in the residual potential between the initial residual potential and the residual potential after repeated use.
A photoconductor for electrophotography, hereinafter also referred to as simply a photoconductor, has a photoconductive layer formed on a conductive substrate. An organic photoconductor, which employs charge-generating or charge-transporting organic compounds for the photoconductive layer, has been researched and developed in recent years. The organic photoconductor has been applied to the fields of copying, printing, and the like, taking advantage of the variety, high productivity, and safety of the organic material.
The organic photoconductor has several functions in electrophotography. The organic photoconductor must maintain a surface charge in darkness, receive light and generate carriers, and transport the generated carriers. The organic photoconductor is classified in two categories, either a single-layered type, or a function-separated multi-layered type. The organic photoconductor of the single-layered type has a photoconductive layer performing all the above functions. The function-separated, multi-layered photoconductor has a photoconductive layer consisting of a charge generation layer and a charge transport layer. The charge generation layer generates charges on exposure to light. The charge transport layer preserves surface charges in darkness. Additionally, the charge transport layer transports the charges generated in the charge generation layer on exposure to light.
Recently, the function-separated, multi-layered organic photoconductor has been chiefly used in the field of electrophotography. The photoconductive layer of the function-separated and multi-layered organic photoconductor consists of a charge generation layer and a charge transport layer. Applying a coating liquid to a conductive substrate forms the charge generation layer. The coating liquid for the charge generation layer is prepared by dispersing charge-generating organic pigment and a resin binder in an organic solvent. Applying a second coating liquid forms the charge transport layer. The coating liquid for the charge transport layer is prepared by dissolving charge-transporting low molecular weight organic compound and a resin binder in an organic solvent.
The properties of a conventional organic photoconductor, however, do not necessarily satisfy all the specified demands. Some of the above stated properties have a strong opportunity for improvement. For example, the property of electric stability in repetitive use has significant room for improvement over the current technology. The repeated and continuous use of a conventional photoconductor in an actual machine causes a variation in electrically charged potential or residual potential, resulting in a deteriorated printing quality.
A factor contributing to this potential variation is accumulation of the generated carriers in the organic photoconductive layer upon light exposure or charge erasing. More specifically, it is believed that the accumulation of the generated carriers originates from the trapping of carriers in the charge generation layer, in the charge transport layer, or at the interface of the two. Another factor is deterioration of the organic material, attributable to radiation, heat, and ozone generated during repetitive use of the photoconductor in an actual machine. Other environmental factors, such as changes in temperature or humidity contribute to the deterioration of the organic material of the photoconductor. Charge-generating materials and charge transporting materials have been recently positively improved. However, neither the means nor the materials have been found by which this problem can be adequately solved.
Moreover, the incorporation of conventional additives to the charge transport layer does not increase the stability of electrically charged potential and residual potential when the photoconductor is used repeatedly. The incorporation of some of the additives to the charge transport layer actually causes a negative effect, decreasing charged potential or increasing residual potential.